1. Field of the Invention
The present invention relates to a circuit board consisting of aluminum nitride (AlN) as a substrate and, more particularly, to a circuit board suitable for forming a module by mounting a semiconductor integrated circuit chip such as an IC or a LSI.
2. Description of the Related Art
Alumina is conventionally mainly used as a substrate of a circuit board for forming a semiconductor integrated circuit chip into a module. As the performance of an integrated circuit chip such as an LSI to be mounted is improved, however, an amount of heat generated by the chip during an operation is increased, and the number of chips to be mounted is limited due to a thermal conductivity of alumina.
For this reason, a circuit board using, as its substrate, BeO having a high thermal conductivity in place of alumina has been used. Since, however, BeO has strong toxicity, its application range as a substrate is limited. Therefore, an AlN sintered product is widely used as a substitute material.
The AlN sintered product is harmless and therefore is not limited in its manufacture, parts formation, and disposal. In addition, the AlN sintered product has a wide thermal conductivity range of 70 to 280 W/m.K and heat radiation properties which can be adjusted from a value 3.5 times that of alumina to a level higher than that of BeO. Therefore, not only a circuit board using an AlN sintered product can realize a packing density higher than that of a circuit board using an alumina substrate, but also a desired thermal conductivity can be imparted in accordance with a high packing density of a semiconductor integrated circuit chip.
Two types of circuit boards are known which have an AlN substrate. A circuit board of the first type is disclosed in Published Unexamined Japanese Patent Application No. 1-223737 (Fukushima et al.). This circuit board has a structure in which a three-layered circuit pattern obtained by sequentially stacking a first layer consisting of at least one member selected from the group consisting of Ti, Cr, Mo, and W, a second layer consisting of Ni or Cu, and a third layer consisting of Au is formed on the surface of an AlN substrate.
A circuit board of the second type is disclosed in Published Unexamined Japanese Patent Application No. 62-182183 (Sasame et al.). In this circuit board, an aluminum oxide layer is formed on the surface of the AlN substrate, and a three-layered circuit pattern formed by sequentially stacking a first layer consisting of Ti, a second layer consisting of Mo or Pt, and a third layer consisting of Au is formed on the aluminum oxide layer.
In each circuit boards having these structure, however, since a bonding strength between the AlN substrate and the circuit pattern is unsatisfactory, the circuit pattern is peeled or disconnected from the substrate surface. In addition, in the AlN substrate, an etching rate changes in accordance with a crystal orientation, and a step is formed between grain boundaries having different crystal orientations. Therefore, when the bonding strength of the circuit pattern with respect to the substrate is unsatisfactory, the circuit pattern is disconnected on the step.